1. Field of the Invention
The present invention relates to an ELID grinding wheel for performing electrolytic dressing while grinding a workpiece, and to an ELID surface grinding apparatus using the grinding wheel.
2. Description of Related Art
In order to grind a high-precision magnetic disk substrate, mainly a plane lapping machine has heretofore been used. This plane lapping machine is the lapping machine of a simultaneous double-sided lapping system. In the machine, gears are formed in the periphery of a fixture to which a workpiece is attached, the fixture meshes with the middle and peripheral gears to rotate itself and the machine, a pressure by an upper cylinder is applied, a loose abrasive material is interposed between a tool called a lapping plate and the workpiece, and lapping is performed by the relative movement of the lapping plate and the workpiece.
Although the above-described conventional lapping apparatus (e.g., the plane lapping machine) has a characteristic that a high machining accuracy can be obtained by relatively simple equipment, but contrarily has the following problems.
(1) Since the losse abrasive is used in the lapping, the lapping is very low ({fraction (1/10)} or less of the grinding removal rate), and the lapping requires much time. Therefore, large-sized equipment is usually used, and a plurality of magnetic disk substrates are simultaneously lapped, but the lapping time per disk is long.
(2) The lapping plate as a standard is preparaed beforehand with a high accuracy, and the workpiece is grinded in conformity with the standard. Therefore, when the accuracy of the lapping plate itself is deteriorated by wear, the re-sharpening is necessary again. In this case, xe2x80x9cmutual lappingxe2x80x9d which is a process for grinding the upper and lower lapping plates with each other is usually performed. Although irregularities are flattened by this means, but squareness cannot be secured with respect to a rotating shaft, and the parallelism of the worked workpiece is deteriorated. Specifically, in a constant-pressure lapping system, truing (mutual lapping) can be performed on the machine. However, even when the flatness of a grinding wheel (lapping plate) is enhanced, the parallelism between a workpiece holding jig and the grinding wheel cannot be adjusted, and the parallelism of both workpiece surfaces (thickness precision) cannot be enhanced by this adjustment.
Furthermore, in recent years, the breakage of a hard disk, and particularly the breakage of a motor have frequently occurred. One of the causes for this lies in the unbalance of the thickness of the magnetic disk substrate. Specifically, there is a problem that the eccentric force generated by the unbalance of the magnetic disk substrate shortens the motor bearing life, and the motor breaks in a short time. Therefore, in order to enhance the reliability of the hard disk, it has strongly been desired that the parallelism (thickness precision) of the magnetic disk substrate be enhanced more than before. However, if this tries to be achieved by the above-described conventional lapping machine, the lapping time is further lengthened, which is impractical and raises a problem.
In order to solve various above-described problems, the present inventors, et al. have devised xe2x80x9cMirror Working Apparatus and Method of Magnetic Disk Substratexe2x80x9d (Japanese Patent Application No. 136198/1998, not laid open) and filed an application. In this invention, as schematically shown in FIG. 1, the apparatus comprises: a metal bond grinding wheel 2 which rotates centering on a vertical shaft center Z1 and includes a horizontal working surface 2a; workpiece holding rotation means 4 which includes a horizontal support surface 4a disposed opposite to the working surface and rotates centering on a vertical shaft center Z2; voltage applying means 6 which uses the metal bond grinding wheel as an anode, and an electrode 6a disposed opposite to the working surface of the metal bond grinding wheel in a non-contact manner as a cathode, and applies a pulse voltage between both electrodes; and electrolytic fluid supply means 8 for supplying a electrolytic fluid to the working surface of the metal bond grinding wheel, and the workpiece holding rotation means 4 brings a disc-shaped magnetic disk substrate 1 or a disc-shaped truing grinding wheel in close contact with the support surface to hold and rotate the substrate or the grinding wheel, and is constituted to be horizontally and vertically movable. The method by this apparatus comprises: (A) plane-working the working surface 2a of the metal bond grinding wheel on the machine; subsequently electrolytic dressing the metal bond grinding wheel; and simultaneously performing (B) grind-working of the support surface 4a of the workpiece holding rotation means and (C) grind-working of the magnetic disk substrate 1 attached to the workpiece holding rotation means alternately on the machine. Additionally, in FIG. 1, numerals 3, 5 denote drive apparatus.
According to this apparatus and method, as compared with the conventional lapping apparatus, the removal rate of the magnetic disk substrate can largely be enhanced, and both surface parallelism (thickness precision) and surface roughness of workpiece can be enhanced more than before.
In the conventional art, the grinding wheel 2 shown in FIG. 1 is integrally molded and the surface 2a is entirely formed into a flat surface without any recess. However, when the grinding wheel 2 is enlarged in size with the enlargement of the workpiece 1 (magnetic disk substrate), in order to manufacture the large-sized grinding wheel with a high accuracy by press molding and sintering, there are many limitations in respect of equipment and there is a problem that manufacture cost is excessively increased.
Moreover, since the surface of the grinding wheel is a flat surface without any recess, an electrolytic fluid is not easily supplied to a gap with the workpiece, and there is a problem that chips and grinding wheel pieces generated by grinding are not easily removed from the grinding wheel surface. Therefore, scratches are generated on the workpiece by the chips or grinding wheel pieces generated during grinding, the supply of the electrolytic fluid partially becomes insufficient and the electrolytic dressing becomes insufficient, and other problems occur.
Furthermore, when the supply amount of the electrolytic fluid is excessively increased to prevent the scratches, the workpiece floats up from the grinding wheel by the pressure of the electrolytic fluid so that the grinding cannot be performed, that is, a so-called hydroplaning phenomenon easily occurs.
The present invention has been developed to solve the above-described problems. Specifically, an object of the present invention is to provide an ELID conductive grinding wheel in which size is easily enlarged, chips and grinding wheel pieces can smoothly be removed, insufficient grind fluid does not easily occur, and hydroplaning phenomenon does not easily occur and to provide an ELID surface grinding apparatus which uses this grinding wheel.
According to the present invention, there is provided an ELID grinding wheel which is a conductive grinding wheel rotating centering on a vertical shaft center and including a horizontal working surface, the grinding wheel comprises a conductive base member of an integrally molded flat plate, and a plurality of fan-shaped segments detachably attached to the base member to entirely constitute the working surface in an annular shape, each segment comprises a base metal directly attached to the base member and a grinding wheel part formed on the surface of the base metal, the grinding wheel part comprises a recessed groove for partitioning the working surface into a plurality of working areas and an electrolytic fluid supply hole for directly supplying an electrolytic fluid to the respective working areas, so that the electrolytic fluid is supplied to each electrolytic supply hole through the base member.
According to the constitution of the present invention, since the conductive grinding wheel comprises the conductive base member of the integrally molded flat plate and a plurality of fan-shaped segments detachably attached to the base member to entirely constitute the annular working surface, the fan-shaped segment is reduced in size, and a large-sized grinding wheel can easily be manufactured using conventional molding equipment and sintering equipment.
Moreover, since the grinding wheel parts of the respective segments include a plurality of working areas partitioned by the recessed grooves, and the electrolytic fluid is directly supplied from the electrolytic supply holes disposed in the respective working areas, the electrolytic fluid flows in a radial one direction from the electrolytic fluid supply hole to the recessed groove surrounding the hole, and the chips and grinding wheel pieces can smoothly be removed with this flow. Furthermore, since it is easy to supply an optimum amount of electrolytic fluid to the respective working areas, the insufficient grind fluid does not easily occur, and the hydroplaning phenomenon by the excessive fluid does not easily occur.
According to a preferred embodiment of the present invention, the respective working areas have substantially the same area, the respective electrolytic fluid supply holes have substantially the same sectional area and the same number of holes are disposed in the respective working areas. According to this constitution, only by supplying substantially the same amount of electrolytic fluid to the respective working areas, the insufficient or excessive part of the grinding fluid can be prevented from being generated over the entire working surface.
Moreover, the base metal is provided with an electrolytic fluid supply port connected to each electrolytic fluid supply hole. Since the electrolytic fluid supply port is disposed, the electrolytic fluid can easily be supplied to each electrolytic fluid supply hole through the base member.
Furthermore, the recessed groove includes a boundary groove extending in the radial direction of the grinding wheel along a boundary line with the adjacent segment, a middle groove disposed between the boundary grooves and extending in the radial direction of the grinding wheel, and an intersecting groove extending to intersect the boundary groove and/or the middle groove. According to this constitution, the chips and grinding wheel pieces having reached the intersecting groove can smoothly be discharged to the outside of the grinding wheel through the boundary groove and/or the middle groove by the action of a centrifugal force.
Moreover, according to the present invention, there is provided an ELID surface grinding apparatus comprising an electrolytic fluid supply apparatus for distributing/supplying a conductive electrolytic fluid to a plurality of electrolytic fluid supply ports of the above-described ELID grinding wheel.
According to the constitution of the present invention, the conductive electrolytic fluid can be distributed/supplied to the plurality of electrolytic fluid supply ports of each ELID grinding wheel from the electrolytic fluid supply apparatus.
According to the preferred embodiment of the present invention, the electrolytic fluid supply apparatus includes an electrolytic fluid chamber disposed in the upper part of the grinding wheel, and a plurality of pipelines for independently connecting the chamber to the respective electrolytic fluid supply ports. According to this constitution, the electrolytic fluid pooled in the electrolytic fluid chamber can be distributed/supplied to the respective electrolytic fluid supply ports by its own weight, and the constitution is suitable especially for the upper grinding wheel.
Moreover, the electrolytic fluid supply apparatus may be constituted of a rotary connector disposed on the rotation centerline of the grinding wheel, and a plurality of pipelines for independently connecting the rotary connector to the respective electrolytic fluid supply ports. According to this constitution, even in the lower grinding wheel, the electrolytic fluid can smoothly be supplied to the respective electrolytic fluid supply ports from the rotary connector.
Other objects and advantageous characteristics of the present invention will be apparent from the following description with reference to the accompanying drawings.